1. Field of the Invention
The present invention relates to an epitaxial wafer production apparatus, a susceptor structure, and an epitaxial wafer production method for the formation of an epitaxial layer on a wafer, and more particularly to an epitaxial wafer production apparatus, a susceptor structure, and an epitaxial wafer production method capable of suppressing the increase in dopant concentration in the peripheral edge portion of the front surface of the epitaxial wafer.
2. Description of the Related Art
A technology is known for the production of a silicon wafer which is free from crystal defects and has the desired resistivity by growing a crystal layer of silicon on the front surface of the wafer. For example, in the case of a wafer with a diameter of 200 mm and a thickness of 0.75 mm, such a silicon crystal layer is an ultra-thin layer with a thickness of about several microns. It is generally called an epitaxial layer, and an apparatus for forming the epitaxial layer called an epitaxial wafer production apparatus. Epitaxial wafer production apparatuses of a single wafer processing type, in which silicon wafers were processed one by one, have been used most often because of high quality of the film produced and easiness of film thickness control.
As shown in FIG. 1, in the epitaxial wafer production apparatus of a single wafer processing type, a susceptor 4 (wafer support) supporting one wafer horizontally is usually provided inside a treatment chamber 2. Further, a lift mechanism for moving a wafer 12 up and down with respect to the susceptor 4 is provided for transporting the wafer 12 on the susceptor 4. The lift mechanism comprises a plurality of lift pins 23 extending through the susceptor 4. The wafer 12 is carried on the upper ends of those lift pins 23, and the wafer 12 is lifted or lowered by moving the lift pins 23 up and down with respect to the susceptor 4. With such a lift mechanism, the wafer 12 carried by the hand of a transport arm and into the chamber 2 is moved on the susceptor 4 or, conversely, transferred from the susceptor 4 to the hand.
Further, the wafer 12 supported on the susceptor 4 has to be heated at a high temperature to grow an epitaxial film. For this reason, heat sources 8, 9 such as a plurality of halogen lamps (IR lamps) are arranged above and below the treatment chamber 2 to heat the susceptor 4 and the wafer 12.
The susceptor 4 is prepared by coating a silicon carbide SiC film on a carbon C substrate and plays a role of a liner base maintaining a uniform temperature over the entire wafer 12 when the wafer 12 is heated. As shown in FIG. 2, a recess called a wafer pocket, which is larger in size than the wafer 12 and has a depth of about 1-2 mm is formed in the upper surface of the susceptor 4 to accommodate, for example, a silicon wafer. The inner surface of the wafer pocket 13 has a tapered configuration so as to be in contact only with the outer peripheral portion of the wafer, and measures are taken to minimize the surface contact of the wafer 12 and the inner surface of the wafer pocket 13. An epitaxial layer composed of a silicon thin film is grown on the surface of the wafer 12 by accommodating the wafer 12 inside the wafer pocket 13 and holding the susceptor 4 in a carrier gas containing a raw material gas at the prescribed temperature. The raw material gas as referred to herein is a silicon source gas and a dopant gas.
A chlorosilane gas such as trichlorosilane SiHCl3 or dichlorosilane SiH2Cl2 is usually used as the silicon gas, and diborane (P type) and phosphine (N type) are usually used as the dopant gas. Those gases are introduced into the chamber together with hydrogen H2 as a carrier gas, and HCl is produced as a byproduct in the course of silicon epitaxy by a thermal CVD reaction on the front surface of the wafer. Therefore, though silicon epitaxy proceeds on the front surface of the wafer, a Si—H—Cl atmosphere is formed on the rear surface of the wafer by an overflow mainly caused by gas diffusion, and emission of dopant species from the rear surface of the wafer occurs in the Si—H—Cl atmosphere.
As a result, the atmosphere comprising dopant species and filling the wafer pocket 13 overflows onto the front surface of the wafer by diffusion, and the dopant concentration in the gas phase locally increases in the outer edge portion of the front surface of the wafer 12. As a result, an effect can be observed of the dopant concentration in the epitaxial layer being increased in the peripheral edge portion of the front surface of the wafer.
In order to prevent the above-described overflow of the atmosphere comprising dopant species and filling the wafer pocket 13 onto the front surface of the wafer, in accordance with the invention described in Japanese Patent Application Laid-open No. 10-223545, a hole 50 passing to the rear surface of the susceptor 4 is provided in the outermost peripheral portion of the wafer pocket 13, as shown in FIG. 16. Thus providing a hole 50 passing from the front surface to the rear surface of the susceptor 4 in the outermost peripheral portion of the wafer pocket 13 makes it possible to prevent the overflow of the atmosphere comprising dopant species and filling the wafer pocket 13 onto the front surface of the wafer 12 because the carrier gas containing the raw material gas and flowing on the front surface side of the susceptor 4 outflows to under the susceptor 4 through the hole 50.
According to the invention described in Japanese Patent Application Laid-open No. 10-223545, when the gas is caused to outflow from the front surface side of the susceptor 4 to under the susceptor 4, the raw material gas such as trichlorosilane SiHCl3 or dichlorosilane SiH2Cl2, which has a high reactivity, and HCl and amorphous Si obtained by decomposition in a gas phase flow out in a large amount together with the carrier gas into a lower chamber 7b. 
Referring to FIG. 1, it is clear that the lower chamber 7b comprises a rotary drive mechanism, a lift mechanism, and a transport arm advancement channel for the wafer 12 and is very difficult to clean. The above-described introduction of the raw material gas and decomposition products results in a significant increase in maintenance time and causes a decrease in productivity. Further, the increase in maintenance cost also cannot be ignored.
Furthermore, because heating is also conducted by the heat source 9 from the side of the lower chamber 7b, contamination of the chamber inner wall degrades the uniformity and reproducibility of heating of the susceptor 4 and also causes a spread in the wafer quality.
The invention of the present application was created to resolve the above-described problems, and it is an object of the present invention to provide an epitaxial wafer production apparatus and a susceptor structure capable of discharging the atmosphere containing dopant species and filling the wafer pocket, without causing the flow of a large quantity of the raw material gas from the front surface side of the susceptor to under the susceptor.
Further, it is another object of the invention of the present application to provide an epitaxial wafer production apparatus and an epitaxial wafer production method which make it difficult for the raw material gas to flow from the front surface side of the susceptor to under the susceptor.